A successor communication scheme of W-CDMA (Wideband-Code Division Multiple Access) and HSPA (High Speed Packet Access), a so-called LTE (Long Term Evolution), has been discussed by W-CDMA standardization group 3GPP (3rd Generation Partnership Project).
In the LTE, it is being discussed that OFDMA (Orthogonal Frequency Division Multiplexing Access) and SC-FDMA (Single-Carrier Frequency Division Multiple Access) are to be applied as radio access schemes to downlinks and uplinks, respectively. See non-patent document 1, for example.
The OFDMA is a scheme where a certain frequency band is segmented into multiple smaller frequency bands (subcarriers) and data is transferred in the individual frequency bands. According to the OFDMA, the subcarriers are densely arranged in such a manner that they partially overlap on a frequency axis without mutual interference, which can realize fast transmission and improve frequency utilization efficiency.
The SC-FDMA is a transmission scheme where a certain frequency band is segmented into frequency bands and the different frequency bands are used among multiple user equipments for transmissions, resulting in reduced interference among the multiple user equipments. According to the SC-FDMA, it has a feature of smaller variations of transmission power, which can save power consumption of user equipments and realize wide coverage.
The LTE is a communication system where one or more physical channels are shared among multiple user equipments in any of uplinks and downlinks. The channels shared among the multiple user equipments are generally called shared channels, which correspond to a PUSCH (Physical Uplink Shared Channel) in the uplinks and a PDSCH (Physical Downlink Shared Channel) in the downlinks in the LTE. Also, the shared channels correspond to a UL-SCH (Uplink Shared Channel) in the uplinks and a DL-SCH (Downlink Shared Channel) in the downlinks as transport channels.
In a communication system using the above-mentioned shared channels, it is necessary to signal which of the user equipments are assigned the shared channels for each subframe (1 ms in the LTE). In the LTE, a control channel used for this signaling is referred to as a PDCCH (Physical Downlink Control Channel), a DL-L1/L2 control channel or DCI (Downlink Control Information). The PDCCH may include downlink scheduling information, an uplink scheduling grant, a transmission power control command bit and so on. See non-patent document 2, for example. Also, the subframe may be called a TTI (Transmission Time Interval).
The downlink scheduling information and the uplink scheduling grant correspond to signaling information for indicating which user equipments are assigned the shared channels. The downlink scheduling information may be called a downlink scheduling grant or downlink assignment information.
In the LTE, a synchronous HARQ is applied to uplinks as a HARQ scheme. In other words, as illustrated in FIG. 1, retransmission of uplink shared channels is conducted at a predefined timing from the first transmission timing and more specifically is conducted at a constant cycle. FIG. 1 illustrates a case where the retransmission of the uplink shared channels is conducted at a cycle of 8 subframes. However, the 8 subframe cycle is simply illustrative, and any other cycle may be applied for the retransmission.
In a mobile communication system, when a user equipment moves from a communicating cell to an adjacent cell, the user equipment conducts handover to switch a communicating base station apparatus. Before the handover, the user equipment measures quality of the adjacent cell being a handover candidate and reports the measurement results to the base station apparatus. The quality may be represented as a received level, a received SINR and so on of a reference signal, for example. The reporting to the base station apparatus is conducted through a measurement report. The base station apparatus determines based on the measurement report whether the user equipment is to conduct the handover, and a handover indication message is transmitted to the user equipment as a handover command.
Here, the handover target cell may be not only a cell having the same frequency in the same system but also a cell having a different frequency in the same system or a cell using a different RAT (Radio Access Technology). The cell using the different RAT generally uses a frequency different from the handover source cell, and inevitably the frequency of the handover target cell would be different from that of the handover source cell.
FIG. 2 schematically illustrates handover between cells using different frequencies. In FIG. 2, a LTE system including mobile communication systems using a first frequency f1 and a second frequency f2 and a WCDMA system using a third frequency f3 different from the first and second frequencies is illustrated. The handover between systems using different frequencies or different RATs is described in non-patent document 2, for example.
Generally, a user equipment includes a single radio signal processing unit and thus cannot transmit and receive signals at different frequencies simultaneously. For this reason, if a cell (different frequency cell) using a frequency different from a camped cell (serving cell) is measured, synchronization with the different frequency is required. Accordingly, a gap duration for measurement (measurement gap) is indicated from the base station apparatus to the user equipment in the measurement of the different frequency cell, and the user equipment measures the different frequency cell in the gap duration. More specifically, for example, the length of the gap duration, an arrival cycle (repetition period) of the gap duration, the frequency of the different frequency cell and so on are indicated to the user equipment under RRC measurement control, and the user equipment conducts the different frequency measurement in the indicated gap duration. The different frequency measurement includes frequency change, capturing of a synchronization channel, quality measurement and so on. The gap may be referred to as a measurement gap, for example. A RRC message being the above RRC measurement control may be referred to as a measurement configuration (MeasConfig). The MeasConfig is a RRC message for indicating a measurement configuration.
FIG. 3 is an image diagram of the gap. In FIG. 3, the length and the gap cycle of the gap duration are set to 6 ms and 40 ms, respectively. The term “different frequency measurement” used herein includes not only searching for the different frequency cell and measuring the quality thereof but also searching for the different RAT cell and measuring the quality thereof.
As stated above, the user equipment conducts the different frequency measurements in the gap duration and accordingly cannot communicate to the base station apparatus serving the camped cell (serving cell) in the gap duration.
Also in the LTE, DRX (Discontinuous Reception) control is applied. The DRX control is applied to the case where the base station apparatus is communicating to the user equipment and there is no data to be communicated, and when the user equipment is in the DRX state, the user equipment periodically or discontinuously receives the PDCCH. In this case, the user equipment only has to receive the PDCCH not at all timings but discontinuously, which can reduce battery power consumption.
FIG. 4 is an image diagram of the DRX control. In FIG. 4, the length of the reception duration is set to 5 ms, and the DRX cycle is set to 1280 ms. The reception duration may be referred to as an ON duration or On-duration.
If semi persistent scheduling is applied, the DRX control may be applied to the case where there is data to be communicated. Since it is presumed that the data is periodically transmitted in the semi persistent scheduling, it is not necessary to receive the PDCCH over all time durations. The semi persistent scheduling is a scheduling scheme presently being discussed to implement VoIP and so on. For downlinks, the base station apparatus (eNB) assigns downlink radio resources (PDSCH) to the user equipment in a fixed manner at a predefined cycle. The downlink radio resource starts at a sub-frame (assignment starting time point), in which downlink scheduling information is transmitted to the user equipment via the PDCCH. Also, for uplinks, the base station apparatus (eNB) assigns uplink radio resources (PUSCH) to the user equipment in a fixed manner at a predefined cycle. The uplink radio resource starts at a sub-frame (assignment starting time point), which is after 4 ms from a subframe when DCI is transmitted to the user equipment via the PDCCH.